<p>
  The volatility skew reveals that for put options, implied volatility is higher for deep OTM options and is decreasing as it moves toward ITM options. For call options, the implied volatility is higher for deep ITM options and is decreasing as it moves toward OTM options. From the demand and supply degree, the skew reflects that investors are more willing to buy deep OTM puts and ITM calls. Why there is volatility skew in the market?
  First, the majority of the equity positions are long. Investors usually have two ways to hedge those long positions risks: Buying downside puts or selling upside calls. The increase in demand create increases in the price of downside puts and decreases in the price of upside calls. The volatility is a reflection of options price. Therefore the volatility of in-the-money put is higher and the volatility of in-the-money call is lower.
  The second reason for volatility skew is that the market moves down faster than it moves up. The downside market move is riskier than the upside move. Thus the price of OTM puts is higher than OTM calls.
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